Scale and dimensions problem importing to Unity
From your question your trying to do 3 things at once. maybe some of these tips will help.

To keep the scale at 1,1,1  make your changes in Edit Mode, not Object Mode.

If your object is not scaled to 1,1,1 you can reset this with:
'Apply Scale'. 'ctrl+A -> scale'

To make your plane flat, select all the vertices and scale them on the Z to zero. 

Same to align any row of vertices, select the row, and scale X to 0.

To put the plane at the Global Position z=0, Set your 3d cursor, and then snap the plane to it.